1. Field of the Invention
The present invention relates to a highly reliable solar cell module and a process for said solar cell module.
2. Related Background Art
Japanese Unexamined Patent Publication No. 36404/1997 discloses a photovoltaic element having a configuration as shown in FIGS. 2(a) and 2(b) FIG. 2(a) is a schematic plan view, viewed from a light receiving face side of the photovoltaic element, and FIG. 2(b) is a schematic cross-sectional view, taken along the A--A' line in FIG. 2(a). Particularly, in FIGS. 2(a) and 2(b), reference numeral 201 indicates a semiconductor element (or a photovoltaic element) whose light receiving face side comprises a power generation region with an upper electrode layer 201' provided on a semiconductor layer (not shown) and a peripheral non-power generation region which is free of said upper electrode layer. On each of the opposite end portions of the non-power generation region, an insulating adhesive body 202 is arranged while being fixed thereto. Reference numeral 203 indicates a collecting electrode comprising a plurality of wires arranged on the power generation region at an equal interval to extend onto the insulating adhesive bodies 202 in the non-power generation region such that their opposite end portions are situated on the insulating adhesive bodies 202. Reference numeral 204 indicates a positive electrode terminal member which is contact-bonded on each adhesive body 202 having the extended end portions of the wires as the collecting electrode 203 situated thereon so as to have electrical connection with the wires as the collecting electrode. Reference numeral 205 indicates a negative electrode terminal member which is fixed to the back face of the photovoltaic element so as to have electrical connection by way of soldering, laser beam welding, or ultrasonic welding.
FIG. 3 is a schematic cross-sectional view illustrating an example of a photovoltaic element string comprising a plurality of photovoltaic elements having such configuration as shown in FIGS. 2(a) and 2(b) which are electrically connected with each other in series. Particularly, FIG. 3 is a schematic cross-sectional view illustrating the constitution of a given, serialized portion of said photovoltaic element string in which a photovoltaic elements 301 (a semiconductor element) and a photovoltaic element 311 (a semiconductor element) are serialized. Specifically, a positive electrode terminal 304 which is electrically connected with a collecting electrode 303 on an insulating adhesive body 302 arranged on a peripheral non-power generation region of a photovoltaic element 301 is extended outside the photovoltaic element 301 and it is electrically connected to a negative electrode terminal 315 arranged at a back face of an adjacent photovoltaic element 311 by means of a solder 306, whereby the photovoltaic element 301 and the photovoltaic element 311 are connected in series. Reference numeral 307 indicates a filler resin which fills the spaces between the two elements while enclosing them. Reference numeral 305 indicates a negative electrode terminal, reference numeral 312 an insulating adhesive body, reference numeral 313 a collecting electrode, and reference numeral 314 a positive electrode terminal.
FIG. 5 is a schematic view illustrating an embodiment of sealing a photovoltaic element string by laminating a plurality of lamination materials to produce a solar cell module by a vacuum lamination method. In FIG. 5, reference numeral 501 indicates a photovoltaic element string comprising a plurality of photovoltaic elements electrically connected with each other, for instance, as shown in FIG. 3. Reference numeral 502 indicates a back side nonwoven glass fiber member which is arranged on the back face side of the photovoltaic element string 501, reference numeral 503 is a surface side nonwoven glass fiber member which is arranged on the light receiving face side of the photovoltaic element string 501, reference numeral 504 is a surface side filler resin, reference numeral 505 is a surface protective film, reference numeral 506 is a back side filler resin, reference numeral 507 is an insulating film, and reference numeral 508 is a back face reinforcing member. The back side nonwoven glass fiber member 502 is used in order to foster deaeration in the back face side of the photovoltaic element string 501 and is also used as a spacer in order to ensure electrical insulation of the photovoltaic element string. The surface side nonwoven glass fiber member 503 is used in order to foster deaeration in the light receiving face side of the photovoltaic element string 501 and also in order to attain an improved surface protective performance in the light receiving face side of the photovoltaic element string.
FIG. 7 [comprising FIGS. 7(a) to 7(c)] and FIG. 8 [comprising FIGS. 8(a) to 8(c)] are schematic top views respectively illustrating an example of a conventional crystalline series photovoltaic element string comprising a plurality of crystalline series photovoltaic element having a small area which are electrically serialized with each other. FIG. 7 [FIGS. 7(a) to 7(c)] and FIG. 8 [FIGS. 8(a) to 8(c)] are top views respectively of a horizontal cross section.
Particularly, FIG. 7(a) shows a crystalline series photovoltaic element 701 shaped as a square 100 mm.times.100 mm in size, and FIG. 7(b) shows a crystalline series photovoltaic element string comprising a plurality of the photovoltaic elements 701 which are spaced at an equal interval of 1.5 mm while being electrically connected with each other in series. In the case of the photovoltaic element string shown FIG. 7(b), as shown in FIG. 7(c), when the area of a given photovoltaic element (a) is A and a sum of the clearances (hatched by oblique lines) between said photovoltaic element (a) and adjacent photovoltaic elements (b) arranged next to the photovoltaic element (a) is B, the ratio of B/A is about 0.061.
FIG. 8(a) shows a crystalline series photovoltaic element 801 shaped in as a square 100 mm.times.100 mm in size with four corners cut off, and FIG. 8(b) shows a crystalline series photovoltaic element string comprising a plurality of the photovoltaic elements 801 which are spaced at an equal interval of 2 mm with respect to the lateral arrangement and at an equal interval of 5 mm with respect to the longitudinal arrangement while being electrically connected with each other in series. In the case of the photovoltaic element string shown FIG. 8(b), as shown in FIG. 8(c), when the area of a given photovoltaic element (a) is A and a sum of the clearances (hatched by oblique lines) between said photovoltaic element (a) and adjacent photovoltaic elements (b) arranged next to the photovoltaic element (a) is B, the ratio of B/A is about 0.157.
By the way, a solar cell module is usually prepared by a vacuum lamination method comprising the steps of stacking at least a resin sheet as a lamination material on each of the opposite sides of a given photovoltaic element string to form a stacked body, subjecting the stacked body to vacuum treatment to sufficiently deaerate the inside thereof, and subjecting the stacked body to thermocompression bonding treatment.
FIG. 6(a) is a schematic view illustrating a photovoltaic element string as an example of the above photovoltaic element string, comprising a plurality of relatively small photovoltaic elements 601 of 100 mm.times.100 mm in size which are spaced at an equal interval (602) while connected in a series.
In the case of preparing a solar cell module using the photovoltaic element string shown in FIG. 6(a) in accordance with the above described vacuum lamination method, the deaeration with respect to each of the photovoltaic elements of the photovoltaic element string in the vacuum treatment step is conducted mainly in directions shown by arrows in FIG. 6(a) by means of attraction. Particularly, in the stacking step in the vacuum lamination method, there is obtained a stacked body in which a resin sheet is stacked on the front side of the photovoltaic element string and another resin sheet stacked on the back side of said string. When the stacked body is subjected to the vacuum treatment, the front side of the stacked body can be sufficiently vacuumed to effectively evacuate air and gas component present therein by means of attraction. This due to the following reasons. The front face (the light receiving face) side of each of the photovoltaic elements constituting the photovoltaic element string is provided with a collecting electrode and the like as previously described with reference to FIGS. 2(a) and 2(b) and therefore, a lot of irregularities based on the collecting electrodes and the like of the photovoltaic elements are present at the front side of the photovoltaic element string. Hence, the resin sheet is not in close contact with the front face side of each of the photovoltaic elements, where air and gas component present in the front side of the stacked body are removed.
On the other hand, the back side of the stacked body has a tendency not to be as sufficiently vacuumed as the front side, for the reasons that the back face of each of the photovoltaic elements constituting the photovoltaic element string is substantially free of irregularities because the back face is provided with merely a bus bar. Therefore, the resin sheet is liable to be ine close contact with the back face due to attraction, where air or gas is occasionally confined. However, such air or gas can be evacuated by means of attraction because the migration distance of air or gas (that is, the distance for air or gas to migrate toward and reach the peripheries where it is removed by means of attraction) is short.
Now, it is difficult to produce a large area crystalline series solar cell module because a large crystalline series photovoltaic element having a large area is difficult produce on an industrial scale. However, in the case of an amorphous silicon solar cell module, it can be optionally designed to have a large area because it is possible to produce a large amorphous silicon photovoltaic element having a large area on an industrial scale. An example of such large area amorphous silicon photovoltaic element, is an amorphous silicon photovoltaic element 350 mm.times.240 mm in size.
FIG. 6(b) shows an example of an amorphous silicon photovoltaic element string comprising a plurality of such large area amorphous silicon photovoltaic elements 603 which are spacedly arranged at an equal interval (604) while connected in series.
In the case of preparing a solar cell module using the photovoltaic element string shown in FIG. 6(b) in accordance with the foregoing vacuum lamination method wherein a stacked body is formed, the stacked body is subjected to a vacuum treatment, and the vacuum-treated stacked body is subjected to thermocompression bonding treatment, in the vacuum treatment step, the front side of the stacked body can be sufficiently vacuumed to effectively evacuate air and gas component present therein as well as in the foregoing case. However, for the back side of the stacked body, when air or gas is present under the photovoltaic element as in the foregoing case, the migration distance for air or gas (that is, the distance for air or gas to migrate toward and reach the peripheries where air or gas, is removed by means of attraction) is fairly long. Because of this, such problems as will be described in the following are liable to occur.
That is, in practice, the preparation of a large area amorphous silicon solar cell module is conducted, for example, in the following manner. Firstly, there is provided an amorphous silicon photovoltaic element string comprising a plurality of amorphous silicon photovoltaic elements having a large area which are electrically serialized as shown in FIG. 6(b). Given surface side lamination materials including a surface side filler resin sheet are stacked on the light receiving face side of the photovoltaic element string and given back side lamination materials including a back side filler resin sheet are stacked on the back face side of the photovoltaic element string to form a stacked body. The stacked body is subjected to vacuum treatment to sufficiently deaerate the inside, followed by a thermocompression bonding treatment. In the vacuum treatment step, it is necessary to evacuate air or gas between the respective lamination materials and between the photovoltaic element string and the lamination materials before the surface side filler resin sheet and the back side filler resin sheet are fused. When the deaeration is insufficient, residual air bubbles remain in the resulting solar cell module. In this case, such air bubbles expand due to a temperature change in heat cycle test or in the surrounding atmosphere to cause peeling among the lamination materials. Particularly, as previously described, the back face of each of the photovoltaic elements constituting the photovoltaic element string is provided with few irregularities and because of this, the back face of the photovoltaic element is liable to be in close contact with the resin upon deaerating the inside of the stacked body in the vacuum treatment step. In the case where the back face of the photovoltaic element is in close contact with the resin, when air (or gas) is present in the portion in close contact, the air is difficult to evacuate. As a result, residual air bubbles remain therein, resulting in such problem as described above in that the air bubbles expand due to a temperature change to cause peeling among the lamination materials.
As previously described, in the case where a photovoltaic element string comprising a plurality of small-sized photovoltaic elements serialized with each other is subjected to lamination treatment by the vacuum lamination method in order to produce a solar cell module, since for each of the photovoltaic elements constituting the photovoltaic element string, the distance between the central portion to the peripheral portion is short, sufficient deaeration can be conducted through the peripheral portions of the photovoltaic element string.
However, in the case of a photovoltaic element string comprising a plurality of large-sized photovoltaic elements connected in series, because each of the photovoltaic elements constituting the photovoltaic element string is the distance from the central portion that to the peripheral portion is fairly long, sufficient deaeration is difficult to conduct only through the peripheral portions of the photovoltaic element string, and residual air bubbles remain in the resulting solar cell module. The solar cell module containing such residual air bubble therein are likely to be inferior in reliability when continuously used over a long period of time under severe outdoor environments with changes in the temperature and humidity.
Incidentally, as previously described with reference to FIG. 5, in the case of sealing a photovoltaic element string by laminating a plurality of lamination materials to produce a solar cell module in accordance with the vacuum lamination method, it is known that a surface side nonwoven glass fiber member 503 is arranged on the light receiving face side of the photovoltaic element string in order to foster the deaeration in the front side and a back side nonwoven glass fiber member 502 is arranged on the back face side of the photovoltaic element string in order to foster the deaeration in the back side.
However, this is problematic in the case of producing a long solar cell module using a long photovoltaic element string. That is, when the long photovoltaic element string is stacked on a long nonwoven glass fiber member which is situated on the back face side, the corners of the photovoltaic elements constituting the photovoltaic element string are caught by the nonwoven glass fiber member, and the nonwoven glass fiber member is broken or the photovoltaic element string is turned up or down. Therefore, an extra step is necessitated in order to eliminate these problems or prevent their occurrence. This situation results in complicating the solar cell module production process.
Now, in order to make a solar cell module have a creep rupture resistance, its lamination material is crossliked by a crosslinking agent usually comprising an organic peroxide. In this case, there is a problem in that when the organic peroxide as the crosslinking agent is heated, the organic peroxide is decomposed while producing radicals, and decomposed residuals become gas components.
In the case of a lamination constitution as shown in FIG. 5 in which the back side filler resin 506 is interposed between the photovoltaic element string 501 and the back face reinforcing member 508, when the back side filler resin 506 is incorporated with an organic peroxide as the crosslinking agent, in the thermocompression treatment, gas based on residuals caused when the organic peroxide is decomposed can remain under the photovoltaic element string. This gas is generated after the back side filler resin 506 is fused. Because of this, even when the nonwoven glass fiber member 502 is used, the nonwoven glass fiber member is impregnated with the fused back side filler resin 506 and therefore, degassing is not effectively provided. In the case of a solar cell module in which a decomposed material of the organic peroxide is remains as an air bubble under the photovoltaic element string, when the solar cell module is continuously exposed to a severe outdoor atmosphere with changes in the temperature and humidity, there is a tendency for the air bubble to grow to cause peeling between the back side filler and the photovoltaic element string.
Separately, in the case where the clearance between each adjacent photovoltaic element of the photovoltaic element string is excessively widened in order foster the deaeration or the degassing, such problems as will be described in the following can occur.
That is, in the case of a solar cell module obtained by resin-sealing a photovoltaic element string on a metal steel plate using such lamination materials as shown in FIG. 5 in accordance with the vacuum lamination method, the substrate of each of the photovoltaic element constituting the photovoltaic element string comprises a metal plate or a wafer plate which is relatively rigid and because of this, the solar cell module has a portion in which the photovoltaic elements are present and another portion in which no photovoltaic elements are present, wherein the rigidity of the former is quite different from that of the latter. In such a solar cell module, when the clearance between each adjacent photovoltaic element is excessively widened, the rigidity of a portion where the connection portion of the adjacent photovoltaic elements (the clearance between the adjacent photovoltaic elements) is situated is smaller than that of a portion where the photovoltaic element is situated. This solar cell module suffer from such problems as will be described in the following. That is, the solar cell module is undesirably bent or curved when it is transported or when it is subjected to roll forming, where the nonwoven glass fiber member enclosed in the filler resin is bulked to blanch or peeling occurs between the nonwoven glass fiber member and the filler resin. When the peeling occurs between the nonwoven glass fiber member and the filler resin, gaps can form, and water can accumulate in the gaps due to dew condensation because of repetition of wetting and cooling, resulting of deterioration in the electric characteristics of the solar cell module.
Further, in a conventional solar cell module comprising a photovoltaic element string, with respect to connection of each adjacent photovoltaic elements of the photovoltaic element string such problems can occur, as well as the ones described in the following.
That is, when each adjacent photovoltaic element of the photovoltaic element string is electrically connected so as to have a diminished clearance between the photovoltaic elements in order to improve the filling efficiency by the filler resin, as shown in FIG. 3, the semiconductor element 311 (the photovoltaic element) is arranged such that it is shifted above the semiconductor element 301 (the photovoltaic element). Thus, the step X with respect to the positions of the adjacent photovoltaic elements becomes large. In the case where a photovoltaic element string comprising a plurality of photovoltaic elements which are electrically serialized with each other in the manner as shown in FIG. 3 is sealed by the transparent resin 307 in order to produce a solar cell module, problems can occur, as will be described in the following. That is, as shown in FIG. 3, the thickness Y of the transparent resin 307 as the filler resin situated on an end portion of the photovoltaic element is partially thinned or a stepped portion is not sufficiently filled, where a residual air bubble is formed. The solar cell module produced in this case can have such problems as will be described in the following. That is, when the solar cell module is continuously exposed to severe outdoor environments over a long period of time, the transparent resin is decomposed mainly due to ultraviolet rays impinged therein. In this case, when a nonwoven glass fiber member is present while being enclosed in the transparent resin, in the portion where the transparent resin is thinned, the nonwoven glass fiber member tends to partially rise to the surface causing a gap. In this gap, water is can accumulate due to dew condensation because of repetition of wetting and cooling, resulting in deterioration in the electrical characteristics of the solar cell module.
In order to prevent the nonwoven glass fiber member from being exposed from the transparent resin, increasing the proportion of the amount of the transparent resin versus that of the nonwoven glass fiber member has been considered . However, this is problematic and is not acceptable in practice.
It is generally recognized that in the case where a solar cell module is installed on a roof of a building by integrating with a roofing member, the temperature inside the solar cell module becomes about 45.degree. C. higher than atmospheric temperature around the solar cell module when it is exposed to 1 SUN (full solar radiation (including direct solar radiation and sky solar radiation) of 100 mW/cm.sup.2). It means that when the atmospheric temperature around the solar cell module is 35.degree. C., the inside temperature of the solar cell module becomes 80.degree. C. Under the condition of such inside temperature of the solar cell module, the surface side coating resin of the solar cell module gradually deteriorates and becomes yellowed. In this case, the quantity of incident light which is impinged through the surface side coating resin to reach the photovoltaic elements is decreased. The extent of the decrease in the light transmittance of the surface side coating resin due to the yellowing is enlarged as the thickness of the surface side coating resin is increased. Thus, there is a tendency that the output power of the solar cell module is decreased as the thickness of the transparent resin is increased.
Besides, it is known that when a resin containing an organic peroxide as a crosslinking agent is sandwiched between a pair of gas-impermeable members, e.g., a glass substrate and a photovoltaic element, and the resin is crosslinked by the organic peroxide, the organic peroxide is decomposed by the application of heat to produce a decomposed residual as gas, where the amount of gas generated is increased as the amount of the resin is increased. Also, when the thickness (that is, the amount) of the transparent resin is increased, air bubbles can form in the light receiving face side.